1. Field of the Invention
The present invention relates to an apparatus and processes for backside thinning of MOSFET devices as a fully packaged die or in a wafer-to-wafer fragment form.
2. Description of Related Art
Advanced semiconductor technology including Ultra Large Scale Integrated circuits (ULSI), Bipolar-CMOS hybrid devices (BICMOS), Microelectronic Mechanical Systems (MEMS), and the like, employ a diversity of semiconductor materials such as, for example, silicon, silicon on insulator, strained silicon junction, silicon germanium, germanium, gallium arsenide, and the like. These advanced semiconductor technologies involve increasingly shrinking feature sizes, thinner gate dielectric film thickness and high-k dielectric constant gate layers combined with the complexity of strained silicon shallow junctions, buried oxide SOI films (with thinner buried oxide films to reduce capacitance), bonded wafer substrates, and the like.
The difficulties of accessing front end of line (FEOL) device structures for device measurements, device chip repair, device design verification, and device electrical and physical characterization, while maintaining both device functionality and integrity, through back-end-of the-line (BEOL) unlayering dictates accessing the device being investigated through a backside, i.e., chip die substrate unlayering. Flip chip die attachment to module substrate further necessitates this backside approach in order to maintain full functionality of the module under investigation.
Conventional techniques for backside access to FEOL devices include global thinning of the silicon substrate, followed by wet etch removal of the buried oxide layer for access to the desired FEOL device, or by reactive ion etch or plasma etch. The wet etch approach may involve, for example, BEOL unlayering via the application of a mixture of de-ionized water and hydrofluoric acid, following by application of cesium hydroxide heated to an elevated temperature to remove substrate layers from the backside of the device for exposing a buried oxide layer for access to FEOL devices. In an alternate conventional backside unlayering approach, focused ion beam microscopy (FIB) is used to unlayer the device from the backside to open a window in the silicon substrate and the buried oxide layer for access to the FEOL device. Alternatively, laser micro chemical (LMC) processing may be applied for removal of silicon or polysilicon materials.
However, these conventional approaches of wet etch, FIB microscopy for backside unlayering or LMC to access FEOL devices are plagued by a number of drawbacks.
In the instance of laser microchemical (LMC) removal, only silicon and polysilicon materials can be selectively removed. For wet etch processing, the uncontrollability of certain wet etch chemistries in BEOL unlayering often results in attack of both the buried oxide layer and underlying layers not intended for removal. Wet chemistries also involve ionic species that are often residually left on the device. The residues of these wet chemistries can cause leakage paths and/or shorts in the device being unlayered. In addition, wet etch chemistries often undercut surrounding layers of the area of the device under investigation, such that these undercuts adversely affect junction regions, alter the strain silicon junctions, as well as change the electrical and physical properties of the device being unlayered. Further, the uncontrolled nature of wet etchants may result in non-uniform removal of the buried oxide layer, leaving behind regions where the buried oxide layer is not completely removed and other areas where device regions are exposed or overetched.
Backside unlayering for access to FEOL devices using conventional FIB microscopy approaches also has several limitations and shortcomings. For instance, the high acceleration beam voltages, such as those of 6 keV to 50 keV, can charge localized regions of the device under investigation such that damage occurs to the thin gate oxides or high-k films. Also, the high atomic number of gallium ions used in FIB associated with the liquid gallium source may interact with the device being processed such that leakage paths and/or shorts occur in the device or alter the threshold voltage device characteristics. The etch chemistries of the gases used in FIB processing, such as, XeF2, Br2, Cl2, can also lead to undesirable leakage paths, shorts, and shifts in threshold voltage characteristics of the device undergoing FIB processing.
Thus, as the thickness of buried oxide films continue to decrease with advanced technologies, such as those devices having buried oxide film thickness ranging from between 1100 Angstroms to 550 Angstroms (or lower), the proximity of the highly energetic accelerating FIB beam to the FEOL device imposes limitations on the ability of opening windows from the backside of the buried oxide for access to the FEOL device. In addition, the heat generated by the high atomic number in ion beam at the site where the FIB beam is incident on the region of the device being processed can easily alter, change, or modify the electrical characteristics, as well as affect the strain silicon junction regions of the FEOL device being processed.
In addition to the above problems associated with conventional backside unlayering, other problems associated with conventional topdown unlayering techniques for access to the FEOL device become even more prominent when high numbers of BEOL interconnection levels, e.g., eight or more copper interconnection layers, are present on the device undergoing topdown processing. Also complicating any attempt for circuit side unlayering for access to FEOL structures is the combination of BEOL low-k (i.e. k<2.5) interlevel dielectric films, such as those with low modulus physical properties, as well as the presence of hard mask film layers employed as etch stop layers during chemical mechanical planarization (CMP) processing. For example, results of a SOI MOSFET after conventional backside unlayering processing via chemical etch removal and heat are shown in FIGS. 1 and 2. The chemical etch unlayering to remove the buried oxide layer of the SOI region results in isotropic chemical etch attack causing undercutting, damage and non planar etch removal, collectively shown as 200, of the MOSFET active implant regions 123. The isotropic chemical etch attack also extends beyond the active silicon implant junction regions to damage 201 the buried oxide layer 126. FIG. 2 illustrates, from a backside view, entire regions of undercut and damaged implant areas of the MOSFET of FIG. 1 as a result of such conventional chemical etch removal unlayering. In addition, conventional backside unlayering processing also commonly attack the liner region between the shallow trench insulator and the active silicon implanted regions. Conventional chemical isotropic etch methods result in selective etching of the circumferential region around interconnection vias extending down to the active silicon implant region. This in turn, results in undercutting and damage 200, 201 of implant areas rendering the SOI MOSFET of FIGS. 1 and 2 non-functional for any subsequent electrical characterization by sub-micron atomic force microscopy contact measurements, non contact capacitive measurements or by sub micron tungsten wire contact probing.
In addition, alternative methods utilizing laser assisted chemical etching for backside thinning are limited to unlayering of silicon films or polysilicon films, and are generally not efficient for unlayering of silicon oxide, buried oxide layers, shallow trench insulator films (typically oxide or tetraorthosilicate films), as well as substrate materials including germanium, gallium arsenide, and silicon germanium materials.
Still other known techniques for access to FEOL devices include those that involve non-contact methods of semiconductor backside analysis such as, but not limited to, emission microscopy, infrared wavelength imaging, light induced voltage alteration (LIVA), thermal induced voltage alteration (TUVA), optical beam induced resistance change (OBIRCH), and optical induced beam current (OBIC). These no contact methods of analysis are dependent on backside thinning of heavily doped substrate materials to permit backside imaging. However, such backside imaging does not require full backside unlayering to the active silicon (i.e., shallow junction implant regions) or the exposure of the tungsten interconnect vias, and as such, electrical characterization thereof cannot be accomplished.
Accordingly, as device geometries continue to shrink in size, further improvements are required for backside unlayering to access smaller FEOL devices for the electrical characterization thereof, while maintaining device integrity, reliability and functionality.